Direct Current (DC) power transmission systems are of interest to use in various situations, for instance when transferring electrical power over long distances. High Voltage Direct Current (HVDC) transmission of power has proved to be an efficient alternative to Alternating Current (AC) transmission in many power transmission situations. Several HVDC transmission lines may furthermore be interconnected to form an HVDC transmission network.
A so-called tapping converter is known to tap power from a DC power line to a three-phase alternating current (AC) power line with the three phases connected in series between the DC power line and ground or a negative DC bus. Each phase is then typically connected to a corresponding AC conductor via at least one transformer, which may be a three-phase transformer.
It is also of interest to connect a DC capacitor in series with the secondary winding of the transformer in some instances. This has the advantage of blocking DC voltages.
An arrangement for tapping power comprising such a DC capacitor is for instance known from US 2013/0070495.
It is of interest to provide a tapping converter that considers one or more of the following objectives:                1) Distribute appropriate AC and DC voltages in converter output voltages of all series connected phases        2) Maintain/set cell capacitor voltages in specific range and allow boost mode operation, where sum of all cell voltages are more than the DC potential,        3) The voltage across the introduced capacitor (DC blocking capacitor) may need to be balanced.        4) Alternate approach of using passive filters to mitigate low order AC side and DC side converter generated harmonics.        